Roll_up / roll_back snark side chain ~17000 tps

Question regarding exits during roll-backs that I might be misunderstanding:

Does this mean you would be able to exit a coin you got in a block that will no longer be part of the chain after rollback (e.g. we roll back to block 42, would we be able to exit a token received in block 44)?
Assuming that data for block 44 is unavailable how would someone be able to prove the exit is valid?